Currently, the second generation global mobile communications technology/third generation global mobile communications technology (2G/3G, 2nd Generation/3rd Generation) network has been widely deployed in many regions, which can be regarded as an overall coverage to a certain degree.
Based on a need of communication development, a standardization organization of the third generation partnership project has launched architecture of a new generation evolved network, where a core network part is a system architecture evolution (SAE) network, an access network part is an evolved universal mobile telecommunications system terrestrial radio access network (E-UTRAN) of a long term evolution (LTE) network. The new generation evolved network has covered some urban areas and traffic hot spot regions.
For ease of description, a network that only provides the SAE/LTE in a PS domain is referred to as a pure PS domain network. For different network bearer mechanisms, mobile communication services may be divided into circuit switching (CS) domain bearer services, such as a voice call, a short message, and a supplementary service, and so on; and packet switching (PS) domain bearer services, including a data service and a session service, where the session service includes a voice over Internet Protocol (VoIP) service.
In a practical application, the voice service may be borne in the CS domain and can also be borne in the PS domain. Based on different network coverage scenarios, during a communication procedure of a user, a problem of needing a fallback from an LTE network to a 2G/3G network might often occur. In the prior art, after a user equipment (UE) is connected to the 2G/3G network, if it finds that a location area is changed, the UE initiates a location update process (which usually takes a few seconds) and can only initiate a voice call or receive a call after location update is completed, which greatly affects user experience.